Medical Imaging Techniques PDF

Biophysics- Applications of Physics in Medical Imaging X-rays CT Scans MRI PET X-rays Ordinary Light X-rays  Very little ordinary light can pass through skin. It’s either absorbed at the s...

Biophysics- Applications of Physics in Medical Imaging X-rays CT Scans MRI PET X-rays Ordinary Light X-rays  Very little ordinary light can pass through skin. It’s either absorbed at the surface or reflected back.  To “See” inside we need a kind of Skin “Light” with more energy. Taking “X-rays”  The patient is placed in front of a X-ray source of X-ray. Tube Most of the X-rays pass through the patient’s body.  A photographic plate is placed on the other side of the patient.  X-rays are absorbed by bone but can pass through skin and soft tissue Photographic Plate X-ray  X-rays that are absorbed in Tube the photographic plate cause chemical changes. Bone  These show as darkened areas when the plate is developed. Soft Tissue X-rays are also partly absorbed by some tissues in the body this creates a more subtle picture. Photographic Pate X-ray Tube Bone Soft Tissue What part of the body do these X-rays show? Answer: A knee  Originality of X-rays In 1895, Wilhelm Conrad Roentgen announced his discovery of X-rays.  He had found that when high-energy electrons hit a material such as glass, the material emitted radiation that penetrated objects.  He called this radiation X-rays. It was shown subsequently that X-rays are short-wavelength, electromagnetic radiation emitted by highly excited atoms. Roentgen showed that X-rays could expose film and produce images of objects in solid containers. Such pictures are possible if the container transmits X-rays more readily than the object inside. A film exposed by the X-rays shows the shadow cast by the object. Within three weeks of Roentgen’s announcement, two French physicians, Oudin and Barthelemy, obtained X-rays of bones in a hand. Since then, X-rays have become one of the most important diagnostic tools in medicine. With current techniques, it is even possible to view internal body organs that are quite transparent to X- rays.  This is done by injecting into the organ a fluid dense to X-rays. The walls of the organ then show up clearly by contrast. Advantages of Basic X-ray Imaging  X-rays are easy to produce.  X-ray machines are relatively cheap.  In controlled doses X-ray images are safe to the patient.  Disadvantages of X-ray Imaging  In large doses X-rays can be harmful to health.  Images are only “Shadow Prints” in one “Plan”. No 3D information.  Imaging of soft tissues and organs is not always very detailed.  CT Scans CT or “CAT” scans take X-ray imaging to “C” stands for “Computed” “T” stands for “ Tomography” In Short…. CT scanners are complex X-ray machines attached to very clever computers using complicated mathematics to build up images of our insides.  The patient is placed on a bed  The scanner (X-ray machine) is the shape of a ring  The patient is slowly moved through the ring as the scan takes place. Looking end on…. X-ray Tube X-rays are produced in an X-ray tube, pass through the patient and are detected by the detector. Patient The scanner rotates the X- ray tube and detector so the patient is scanned from all angles. X -ray detector   There are no photographic plates in CT scanners.  There are no photographic plates in CT Ribs scanners.  All images are created by computers using the information they receive from the X-ray detector Spine The image produced is like a “Slice” through the body.  Advantages of CT Scans  Images are like “Slices”.  Compared other scanners (MRI and PET) CT machines are quite cheap.  Disadvantages of CT  Still use X-rays that can damage healthy tissues (in large doses).  Imaging of soft tissues is improved but still not always as detailed as doctors require.  MRI What do the letters stand for? M………... Magnetic R………… Resonance I…………. Imaging MRI scanners do not use X-rays.  MRI Explained...  Your science studies have shown you that your body is made up of living cells...  Which are made up of molecules …  Which are made up of atoms...  The simplest atom is… 1 electron Hydrogen It’s nucleus contains just one proton 1 proton  In the 1940’s physicists discovered that the nuclei of some atoms have a property called “SPIN”… Like a vibrating This causes the nucleus act like a tiny magnet….  After many years of investigation physicists found they could affect the tiny nuclear magnets of hydrogen atoms using very strong magnets and radio waves…  Bring the hydrogen atom in the magnets. Watch how the magnetic field affects the tiny nuclear magnets.  The nuclear magnets can line up in two possible radio waves directions tiny pulse of radio waves A pulse of radio waves can cause some of the nuclear magnets absorb energy and “flip” S N This high energy situation cannot be sustained for long. Many will “flip” back. When this happens energy is released as a tiny pulse of radio waves !!! This tiny pulse of radio waves that can be detected and analysed.  The timing, and the energy of these signals, reveals information about the Hydrogen atoms and tiny pulse of radio waves what types of molecules they are attached to.  So what has all this got to do with looking inside your body?  What is your body mostly made of?  What is the chemical name of water? H2O  Hydrogen in the most abundant element in your body (approx. 63% of all the atoms are H) Organic molecules that make up tissues like FAT MUSCLE etc... Contain a large number of Hydrogen atoms It took physicists over 40 years to turn their discovery of nuclear magnets into images of the human body. But the results are amazing. All this from controlling the magnetic properties of hydrogen nuclei !  Using an MRI Scanner… MRI Scanner  The patient is placed on a bed Patient and then moved into a large hollow tube. Bed  The tube contains a very powerful magnet.  Most MRI scanners use Magnets  An electric current passes through a massive coil made of a special “superconducting” material.  This creates a very strong magnet (20000 times stronger than earths magnetic field). This may seem like a really easy way to create a strong magnet but there is a problem !!!!  Superconducting materials only work correctly when they are really cold… But not just cold like freezer temperatures… Can you guess how cold? Degrees Celsius That’s colder than on the surface of Pluto! To achieve these temperatures the superconducting coils need to sit down in a container filled with… Thankfully the patient is insulated from this extremely low temperature whilst inside the magnet.  The magnet used is unbelievably strong!  Stand 1m away with a large spanner in your hand. You would not be able to hold on to it.  Patients have to remove all metallic objects and credit cards.  But Patients may have metal objects inside their bodies. Patients may be asked the following questions: Have you ever worked in the army or metal working industry? Metal fragments (especially in the eye) could become dislodged Do you have a pacemaker? If yes you cannot have an MRI scan Do you have any dental implants? Some could become magnetised Do you have any metal pins in your body? Some could become magnetised and need to be checked that they will hold in place during the scan With the patient safety check complete the scan can begin.  The part of the body to be scanned is placed in the centre of the primary magnet.  The magnet field produced has to be very steady X and strong.  This field causes the Hydrogen nuclei in the patients body to line up with the field.  Three further coils are embedded into the tube….GRADIENT MAGNETS… these are used to fine tune the magnetic field so particular body parts and tissue types can be focused on. X  The patient will know when these magnets are switched on…they can make a loud banging noise.  More coils provide a pulse of radio waves that cause some of the “nuclear magnets” to flip.  The machine waits and records any radio signals that are then emitted by the patients body. This information is sent to a computer which uses it to build up an image ….  CT compared to MRI  CT scanners scan a patient in “Slices” but the angles of the slices depend on how the patient is positioned in the machine.  MRI scanners scan a whole section of the body then the doctor can request to view a slice of the patient at any angle.  MRI scans can reveal a lot more detail. View an MRI scan from any angle.. Back Pain Are MRI Scans Safe?  Research has failed to show up any risk to health  Patients do not feel a thing….not even a tingle!  Scans typically take 30 minutes  Staying still and putting up with clanging noises are the only discomforts a patient has to suffer! A further group of people may find it impossible to have an MRI scan….! What is the name of the condition that causes a fear of… “Claustrophobia” Many claustrophobics cannot have MRI scans Positron Emission Tomography (PET)  What is PET  PET is a noninvasive, diagnostic imaging technique for measuring the metabolic activity of cells in the human body.  It was developed in the middle 1970s and it was the first scanning method to give functional information about the brain. Htt://www.nucmed.buffalo.edu/petdef.htm  What is a Positron?  A Positron is an anti-matter electron, it is identical in mass but has an apposite charge of +1.  Positron can come from different number of sources, but for PET they are produced by nuclear decay.  Nuclear decay is basically when unstable nuclei are produced in a cyclotron by bombarding the target material with protons, and as a result a neutron is released. 18O + Proton => 18F + Neutron  The obtained 18F is a positron emitter  Annihilation of a positron and electron  The positron will encounter an electron and completely annihilate each other resulting in converting all their masses into energy.  This is the result of two photons, or gamma-rays. http://www.nucmed.buffalo.edu/positron.htm  How do we detect photons (gamma-rays)? PET detects these photons with a PET camera which allows to determine where they came from, where the nucleus was when it decayed, and also knowing where the nucleus goes in the body.  What are some of the uses for PET  Patients with conditions affecting the brain  Heart  Certain types of Cancer  Alzheimer’s disease  Some neurological disorders Htt://www.nucmed.buffalo.edu/petdef.htm  How does it work?  Before the examination begins, a radioactive substance is produced in a machine called a cyclotron and attached, or tagged, to a natural body compound, most commonly glucose, but sometimes water or ammonia.  Once this substance is administered to the patient, the radioactivity localizes in the appropriate areas of the body and is detected by the PET scanner.  Different colors or degrees of brightness on a PET image represent different levels of tissue or organ function.  For example, because healthy tissue uses glucose for energy, it accumulates some of the tagged glucose, which will show up on the PET images.  However, cancerous tissue, which uses more glucose than normal tissue, will accumulate more of the substance and appear brighter than normal tissue on the PET images.  How is it performed?  A nurse or technologist will take you into a special injection room, where the radioactive substance is administered as an intravenous injection (although in some cases, it will be given through an existing intravenous line or inhaled as a gas).  It will then take approximately 30 to 90 minutes for the substance to travel through your body and accumulate in the tissue under study.  During this time, you will be asked to rest quietly and avoid significant movement or talking, which may alter the localization of the administered substance.  After that time, scanning begins. This may take 30 to 45 minutes.  How is it performed? Some patients, specifically those with heart disease, may undergo a stress test in which PET scans are obtained while they are at rest and again after undergoing the administration of a pharmaceutical to alter the blood flow to the heart. Usually, there are no restrictions on daily routine after the test, although you should drink plenty of fluids to wash out the radioactive substance from your body.